1 ==================================================== 2 IN-KERNEL CACHE OBJECT REPRESENTATION AND MANAGEMENT 3 ==================================================== 4 5By: David Howells <dhowells@redhat.com> 6 7Contents: 8 9 (*) Representation 10 11 (*) Object management state machine. 12 13 - Provision of cpu time. 14 - Locking simplification. 15 16 (*) The set of states. 17 18 (*) The set of events. 19 20 21============== 22REPRESENTATION 23============== 24 25FS-Cache maintains an in-kernel representation of each object that a netfs is 26currently interested in. Such objects are represented by the fscache_cookie 27struct and are referred to as cookies. 28 29FS-Cache also maintains a separate in-kernel representation of the objects that 30a cache backend is currently actively caching. Such objects are represented by 31the fscache_object struct. The cache backends allocate these upon request, and 32are expected to embed them in their own representations. These are referred to 33as objects. 34 35There is a 1:N relationship between cookies and objects. A cookie may be 36represented by multiple objects - an index may exist in more than one cache - 37or even by no objects (it may not be cached). 38 39Furthermore, both cookies and objects are hierarchical. The two hierarchies 40correspond, but the cookies tree is a superset of the union of the object trees 41of multiple caches: 42 43 NETFS INDEX TREE : CACHE 1 : CACHE 2 44 : : 45 : +-----------+ : 46 +----------->| IObject | : 47 +-----------+ | : +-----------+ : 48 | ICookie |-------+ : | : 49 +-----------+ | : | : +-----------+ 50 | +------------------------------>| IObject | 51 | : | : +-----------+ 52 | : V : | 53 | : +-----------+ : | 54 V +----------->| IObject | : | 55 +-----------+ | : +-----------+ : | 56 | ICookie |-------+ : | : V 57 +-----------+ | : | : +-----------+ 58 | +------------------------------>| IObject | 59 +-----+-----+ : | : +-----------+ 60 | | : | : | 61 V | : V : | 62 +-----------+ | : +-----------+ : | 63 | ICookie |------------------------->| IObject | : | 64 +-----------+ | : +-----------+ : | 65 | V : | : V 66 | +-----------+ : | : +-----------+ 67 | | ICookie |-------------------------------->| IObject | 68 | +-----------+ : | : +-----------+ 69 V | : V : | 70 +-----------+ | : +-----------+ : | 71 | DCookie |------------------------->| DObject | : | 72 +-----------+ | : +-----------+ : | 73 | : : | 74 +-------+-------+ : : | 75 | | : : | 76 V V : : V 77 +-----------+ +-----------+ : : +-----------+ 78 | DCookie | | DCookie |------------------------>| DObject | 79 +-----------+ +-----------+ : : +-----------+ 80 : : 81 82In the above illustration, ICookie and IObject represent indices and DCookie 83and DObject represent data storage objects. Indices may have representation in 84multiple caches, but currently, non-index objects may not. Objects of any type 85may also be entirely unrepresented. 86 87As far as the netfs API goes, the netfs is only actually permitted to see 88pointers to the cookies. The cookies themselves and any objects attached to 89those cookies are hidden from it. 90 91 92=============================== 93OBJECT MANAGEMENT STATE MACHINE 94=============================== 95 96Within FS-Cache, each active object is managed by its own individual state 97machine. The state for an object is kept in the fscache_object struct, in 98object->state. A cookie may point to a set of objects that are in different 99states. 100 101Each state has an action associated with it that is invoked when the machine 102wakes up in that state. There are four logical sets of states: 103 104 (1) Preparation: states that wait for the parent objects to become ready. The 105 representations are hierarchical, and it is expected that an object must 106 be created or accessed with respect to its parent object. 107 108 (2) Initialisation: states that perform lookups in the cache and validate 109 what's found and that create on disk any missing metadata. 110 111 (3) Normal running: states that allow netfs operations on objects to proceed 112 and that update the state of objects. 113 114 (4) Termination: states that detach objects from their netfs cookies, that 115 delete objects from disk, that handle disk and system errors and that free 116 up in-memory resources. 117 118 119In most cases, transitioning between states is in response to signalled events. 120When a state has finished processing, it will usually set the mask of events in 121which it is interested (object->event_mask) and relinquish the worker thread. 122Then when an event is raised (by calling fscache_raise_event()), if the event 123is not masked, the object will be queued for processing (by calling 124fscache_enqueue_object()). 125 126 127PROVISION OF CPU TIME 128--------------------- 129 130The work to be done by the various states is given CPU time by the threads of 131the slow work facility (see Documentation/slow-work.txt). This is used in 132preference to the workqueue facility because: 133 134 (1) Threads may be completely occupied for very long periods of time by a 135 particular work item. These state actions may be doing sequences of 136 synchronous, journalled disk accesses (lookup, mkdir, create, setxattr, 137 getxattr, truncate, unlink, rmdir, rename). 138 139 (2) Threads may do little actual work, but may rather spend a lot of time 140 sleeping on I/O. This means that single-threaded and 1-per-CPU-threaded 141 workqueues don't necessarily have the right numbers of threads. 142 143 144LOCKING SIMPLIFICATION 145---------------------- 146 147Because only one worker thread may be operating on any particular object's 148state machine at once, this simplifies the locking, particularly with respect 149to disconnecting the netfs's representation of a cache object (fscache_cookie) 150from the cache backend's representation (fscache_object) - which may be 151requested from either end. 152 153 154================= 155THE SET OF STATES 156================= 157 158The object state machine has a set of states that it can be in. There are 159preparation states in which the object sets itself up and waits for its parent 160object to transit to a state that allows access to its children: 161 162 (1) State FSCACHE_OBJECT_INIT. 163 164 Initialise the object and wait for the parent object to become active. In 165 the cache, it is expected that it will not be possible to look an object 166 up from the parent object, until that parent object itself has been looked 167 up. 168 169There are initialisation states in which the object sets itself up and accesses 170disk for the object metadata: 171 172 (2) State FSCACHE_OBJECT_LOOKING_UP. 173 174 Look up the object on disk, using the parent as a starting point. 175 FS-Cache expects the cache backend to probe the cache to see whether this 176 object is represented there, and if it is, to see if it's valid (coherency 177 management). 178 179 The cache should call fscache_object_lookup_negative() to indicate lookup 180 failure for whatever reason, and should call fscache_obtained_object() to 181 indicate success. 182 183 At the completion of lookup, FS-Cache will let the netfs go ahead with 184 read operations, no matter whether the file is yet cached. If not yet 185 cached, read operations will be immediately rejected with ENODATA until 186 the first known page is uncached - as to that point there can be no data 187 to be read out of the cache for that file that isn't currently also held 188 in the pagecache. 189 190 (3) State FSCACHE_OBJECT_CREATING. 191 192 Create an object on disk, using the parent as a starting point. This 193 happens if the lookup failed to find the object, or if the object's 194 coherency data indicated what's on disk is out of date. In this state, 195 FS-Cache expects the cache to create 196 197 The cache should call fscache_obtained_object() if creation completes 198 successfully, fscache_object_lookup_negative() otherwise. 199 200 At the completion of creation, FS-Cache will start processing write 201 operations the netfs has queued for an object. If creation failed, the 202 write ops will be transparently discarded, and nothing recorded in the 203 cache. 204 205There are some normal running states in which the object spends its time 206servicing netfs requests: 207 208 (4) State FSCACHE_OBJECT_AVAILABLE. 209 210 A transient state in which pending operations are started, child objects 211 are permitted to advance from FSCACHE_OBJECT_INIT state, and temporary 212 lookup data is freed. 213 214 (5) State FSCACHE_OBJECT_ACTIVE. 215 216 The normal running state. In this state, requests the netfs makes will be 217 passed on to the cache. 218 219 (6) State FSCACHE_OBJECT_UPDATING. 220 221 The state machine comes here to update the object in the cache from the 222 netfs's records. This involves updating the auxiliary data that is used 223 to maintain coherency. 224 225And there are terminal states in which an object cleans itself up, deallocates 226memory and potentially deletes stuff from disk: 227 228 (7) State FSCACHE_OBJECT_LC_DYING. 229 230 The object comes here if it is dying because of a lookup or creation 231 error. This would be due to a disk error or system error of some sort. 232 Temporary data is cleaned up, and the parent is released. 233 234 (8) State FSCACHE_OBJECT_DYING. 235 236 The object comes here if it is dying due to an error, because its parent 237 cookie has been relinquished by the netfs or because the cache is being 238 withdrawn. 239 240 Any child objects waiting on this one are given CPU time so that they too 241 can destroy themselves. This object waits for all its children to go away 242 before advancing to the next state. 243 244 (9) State FSCACHE_OBJECT_ABORT_INIT. 245 246 The object comes to this state if it was waiting on its parent in 247 FSCACHE_OBJECT_INIT, but its parent died. The object will destroy itself 248 so that the parent may proceed from the FSCACHE_OBJECT_DYING state. 249 250(10) State FSCACHE_OBJECT_RELEASING. 251(11) State FSCACHE_OBJECT_RECYCLING. 252 253 The object comes to one of these two states when dying once it is rid of 254 all its children, if it is dying because the netfs relinquished its 255 cookie. In the first state, the cached data is expected to persist, and 256 in the second it will be deleted. 257 258(12) State FSCACHE_OBJECT_WITHDRAWING. 259 260 The object transits to this state if the cache decides it wants to 261 withdraw the object from service, perhaps to make space, but also due to 262 error or just because the whole cache is being withdrawn. 263 264(13) State FSCACHE_OBJECT_DEAD. 265 266 The object transits to this state when the in-memory object record is 267 ready to be deleted. The object processor shouldn't ever see an object in 268 this state. 269 270 271THE SET OF EVENTS 272----------------- 273 274There are a number of events that can be raised to an object state machine: 275 276 (*) FSCACHE_OBJECT_EV_UPDATE 277 278 The netfs requested that an object be updated. The state machine will ask 279 the cache backend to update the object, and the cache backend will ask the 280 netfs for details of the change through its cookie definition ops. 281 282 (*) FSCACHE_OBJECT_EV_CLEARED 283 284 This is signalled in two circumstances: 285 286 (a) when an object's last child object is dropped and 287 288 (b) when the last operation outstanding on an object is completed. 289 290 This is used to proceed from the dying state. 291 292 (*) FSCACHE_OBJECT_EV_ERROR 293 294 This is signalled when an I/O error occurs during the processing of some 295 object. 296 297 (*) FSCACHE_OBJECT_EV_RELEASE 298 (*) FSCACHE_OBJECT_EV_RETIRE 299 300 These are signalled when the netfs relinquishes a cookie it was using. 301 The event selected depends on whether the netfs asks for the backing 302 object to be retired (deleted) or retained. 303 304 (*) FSCACHE_OBJECT_EV_WITHDRAW 305 306 This is signalled when the cache backend wants to withdraw an object. 307 This means that the object will have to be detached from the netfs's 308 cookie. 309 310Because the withdrawing releasing/retiring events are all handled by the object 311state machine, it doesn't matter if there's a collision with both ends trying 312to sever the connection at the same time. The state machine can just pick 313which one it wants to honour, and that effects the other. 314